Positioning devices in gantry type of construction—in which a crossbeam is movably supported between two parallel linear guides, and a functional element is movably supported on the crossbeam with the aid of a further linear guide, so that this functional element may be freely positioned in a plane between the two parallel linear guides—have been in the state of the art for a long time. For example, a gripper of an automatic pick-and-place machine, a laser of a laser machining center, or perhaps a probe system of a coordinate measuring machine are possible as a functional element.
In all these practical applications, of which there are still more for gantry-type positioning devices, the most precise possible positioning of the functional element plays a crucial role. Therefore, often a great effort is expended to position the functional element as accurately as possible using the most precise position-measuring devices possible.
Position-measuring devices made up of scales having assigned scanning heads are based on the scanning of periodic structures on the scale. If the scale and the scanning head move relative to each other, the scanning head generates periodic signals, from which it is possible to derive the relative shift. In this context, the structures on the scale may also be implemented such that absolute position information is able to be read. This is managed either with reference marks or with absolutely coded tracks on the scale. Such position-measuring devices are used extensively in the realm of positioning devices, so that a more precise description of the functioning method is unnecessary.
For positioning devices in gantry type of construction, both European Patent No. 0 082 441 and U.S. Pat. No. 6,949,733 recommend the use of position-measuring devices based on the scanning of graduations disposed on a scale. Besides bearing the customary incremental track having a great number of graduated scale lines disposed transversely to the measuring direction (hereinafter referred to as a measuring track), the scales also bear an additional measuring structure having graduated scale lines in the measuring direction which are a few, but instead extend over the entire measuring length (hereinafter referred to as a straightness track). Because of the possibility of deriving small position deviations transversely to the actual measuring direction from this straightness track, such scales are also known as 1D± scales. In both documents mentioned above, such 1D± scales are used to measure guiding errors and tilts and to take them into consideration in the positioning. In part, errors are also taken into account which are of the sort that come about due to thermal expansion of individual components of the positioning device. However, it is not possible to completely measure and compensate for the thermal expansion of the crossbeam using any of the arrangements indicated in these documents. It is precisely this crossbeam, however, which is usually especially affected by such expansions when, for example, the transverse axis is driven by a linear motor that is disposed on the crossbeam, and whose waste heat heats up the crossbeam.
The crossbeam with linear guide is also denoted hereinafter as transverse guide.
1D± scales and measuring devices based on them are described in greater detail in German Published Patent Application No 10 2005 023 984.
In addition, it is described in German Published Patent Application No. 42 12 970 that, to compensate for thermal expansions when measuring a position, it is important to skillfully select the fixed point of a scale which is otherwise float-mounted, e.g., mounted movably on its substrate with the aid of a flexible adhesive layer. At this fixed point, the scale is firmly joined to its support, so that the scale is moved along in response to thermal displacement of this fixed point.